Meet Kristin Cavallari's Son Camden Jack

"I like having this life that I'm responsible for," says Cavallari, 25. "It's exciting and so rewarding," but the Laguna Beach and Hills reality star admits, "it's also a little scary!"

In this week’s PEOPLE, Kristin Cavallari and her fiancé, Chicago Bears quarterback Jay Cutler, open up their Chicago home to introduce 6-week-old son Camden Jack and reveal how life changed instantly with their “new family.”

“I like having this life that I’m responsible for,” Cavallari, 25, tells PEOPLE. “It’s exciting and so rewarding,” but the Laguna Beach and Hills reality star admits, “it’s also a little scary!”

After a relatively easy pregnancy (“I was lucky,” she says), her new priority became even clearer once she held Camden in her arms for the first time.

“It was the most incredible moment I’ve ever felt,” she explains. “With Camden, I’ve realized what really matters. We’re a family.”

And the first thing Cutler, 29, noticed when gazing down at his newborn son? “He pointed out right away how big his feet are! And his hands were massive — he’ll sure be able to hold a football.”

For exclusive photos of Camden — plus details about Cavallari’s life as a new mom and if more kids are in their future — pick up this week’s issue of PEOPLE, on newsstands Friday.

— Jennifer Garcia